Ring opening polymerization of lactams is an important route to commercial polyamides such as nylon 6 and nylon 12. Initially, nylon 6 was produced by the ring-opening polymerization (“ROP”) of ε-caprolactam in the presence of small amounts of acid or water. Subsequently, anionic ROP was preferred, typically with sodium lactamate salt catalysts and isocyanate/lactam adducts as initiators. Grignard salts of ε-caprolactam have also been used as catalysts for ROP of lactams [K. Ueda et al., Polymer Journal (1996), 28(5), 446-451], as have poly(aminophosphazenes) and protophosphatranes [W. Memeger, Jr. et al., Macromolecules (1996), 29, 6475-6480]. More information about the production of polyamides from anionic ROP of lactams is available in a variety of sources that include, for example, (i) H. Sekiguchi, “Lactams and Cyclic Imides,” in Ring-Opening Polymerization, K. J. Ivin and T. Saegusa, eds., Elsevier Applied Science Publishers Ltd., Essex, England, vol. 2 (1984), Ch. 12, 809-918; (ii) K. Udipi et al., Polymer (1997), 38(4), 927-938; (iii) R. S. Davé et al., Ibid., 939-947; (iv) R. S. Davé et al., Ibid., 949-954; and (v) J. Sebenda, Prog. Polym. Sci. (1978), 6, 123-167.
Macrocyclic amides can be formed as minor byproducts in the production of linear polyamides. For example, when hexamethylene diamine and adipic acid are polymerized to make nylon 66, cyclic species are produced at a level of about 1.5%: cyclic unimer, cyclic dimer, cyclic trimer, and traces of higher cyclic oligomers. Macrocyclic aromatic amides are also known and have been synthesized under dilute reaction conditions. Flexible linkages and kinks increase the propensity to form macrocyclics [W. Memeger, Jr., “Macrocyclic Aramids” in Polymeric Materials Encyclopedia, J. C. Salamone editor-in-chief, CRC Press, Inc., Boca Raton, Fla. (1996), 3873-3882].
Linear polyamides may be fabricated into articles of manufacture by a number of known techniques including extrusion, compression molding, and injection molding. However, lactams and macrocyclic amides have unique properties that make them attractive as matrices for engineering thermoplastic composites. The desirable properties stem from the fact that lactams and macrocyclic amides exhibit low melt viscosity, allowing them easily to impregnate a dense fibrous preform followed by polymerization to polyamides. Upon melting and in the presence of an appropriate catalyst, polymerization and crystallization can occur virtually isothermally.
There thus remains a need for an effective and efficient high-temperature process for preparing linear polyamides from cyclic amides.